The present invention relates generally to electronic components and more particularly to surge protection and gas tube surge arresters.
The demand for devices that protect sensitive electronic components from overvoltage surges is increasing. There are different devices on the market for this purpose. Certain of these devices are better suited for certain applications.
There are generally two surge protection classifications, each including different types of devices. One classification of surge protection devices is the “crowbar” classification. Crowbar devices include air gaps, carbon blocks, silicon controlled rectifiers (“SCR's”), voltage variable material (“VVM”) devices and gas tube surge arresters, the subject of the present invention. Another classification of surge protection devices is the “clamping” classification. Clamping devices include zener or avalanche diodes and metal oxide varisters (“MOV's”).
“Clamping” devices limit the voltage transient to a specified level by varying an internal resistance based on the applied voltage. The clamping devices themselves absorb the energy of the transient. Clamping devices have relatively quick response times but are relatively limited in ability to withstand high current levels.
Generally, a “crowbar” device limits the energy delivered to the protected circuit by abruptly changing from a high impedance state a low impedance state in response to an elevated voltage level. After being subjected to a sufficient voltage level the crowbar device, which is normally nonconductive, begins to conduct. While conducting, the arc voltage across the crowbar device remains relatively low (e.g., at or below 15 volts for gas discharge tube curve as shown below in FIG. 3. The majority of the transient's power is dissipated to ground or to the resistive elements of the circuit and not to the portion of the circuit intended to be protected by the crowbar device or gas tube surge arresters. Such power dissipation renders gas tube surge arresters able to withstand and protect loads from higher voltage and/or higher current levels for a greater duration of time than clamping devices.
Referring to FIG. 1, one known gas tube surge arrester 10 includes two electrodes 12 and 14 that are fitted with a hollow cylindrical ceramic insulator 16. Inside the insulator 16, inner surfaces of electrodes 12 and 14 are coated with an activating compound. Referring to FIGS. 2A and 2B, another known gas tube surge arrester 20 includes the two outer electrodes 12 and 14 that are fitted with two ceramic insulators 20 and 22, which are separated by a third electrode 24. Both arresters 10 and 20 house a gas, such as argon or neon. The activating compound aids in making the gas conductive upon an overvoltage transient event.
Operating parameters for gas tube surge arresters include: (i) static or DC sparkover voltage, (ii) dynamic or surge sparkover voltage, (iii) extinguishing voltage, (iv) glow voltage, (v) current-carrying capacity under alternating current and (vi) unipolar pulsed current. Those operating parameters can be effected by various factors, such as: (i) the structural layout of the electrodes, (ii) the type of gas used, (iii) the pressure at which the gas is maintained within the arrester, (iv) the configuration of one or more ignition strip within the arrester, and (v) the activating compound disposed on the active surfaces of the electrodes.
The activating compounds can include multiple components. For example, one known compound includes three components, namely, aluminum, sodium bromide and barium titanate. While this compound is useable, a need exists for new activating compounds that attempt to improve the operating parameters of gas tube surge arresters, such as the operating parameters listed above.